It is known to drive various automobile accessory assemblies, for example the water pump, the generator, the fan for cooling the coolant, the power steering pump, and the compressor, by the vehicle engine. This is done by a driving pulley actuated by the engine shaft of the motor vehicle which drives an endless drive belt operating the accessory assemblies through driven pulleys.
Different states of operation of the engine are known in which the belt tension increases pulse-like generating a growing torque on the pivot arm which, in response to varying tension of the belt, may deflect in a direction opposite to a belt tensioning direction. In order to prevent the unnecessary sliding friction between the pivotal arm and the stationary case, the prior art teaches numerous resiliently deformable elements which are disposed between the pivotal arm and the stationary house. Usually, such deformable elements are bushings made of friction materials. Increasing the number of bushings leads to complicated kinematics, which may result in structural damages to the tensioner. Typically, the tensioner may have increased undesirable wear of many frictional parts as a result of tensioning forces exerted by new elements. This brings about tension peaks in the belt, which can exceed the permissible belt tension. Ultimately, all of the above described defects can cumulatively cause undesirably large excursions of the pivotal arm.
Recognizing this problem, the prior art has designed numerous structures of belt tensioners which allow reduced dynamic loads on the main elements of the tensioner. Two basic premises are usually considered during design and assembly of the belt tensioner. First, the ideal assembly has to be tight enough to have all the clearances removed. Second, the ideal assembly must be sufficiently loose to avoid locking up the tensioner.
The structure that has been previously designed is shown in FIGS. 1 and 1A, and as a result of practical observations, is illustrative of the problems inherent in this type of tensioner. The belt tensioner has a disk shaped armplate 5' serving a dual function. First, the armplate 5' holds the assembled tensioner together. Second, the plate removes the clearances between the various components of the tensioner during assembly. The armplate has a wear bushing 4' between it and a tensioner arm 1' as a result of the relative rotational movement that exists between these two components when the tensioner is in operation. Typically, the wear bushing and the armplate have had the same outer and inner diameters. Such dimensions have caused assembly problems due to the dimensional variation of a plurality of components that must be mated together to build a tensioner.
The plate, as known in the art, is customarily attached to the tensioner by one of the following methods:
According to the first method, the armplate 5' is pressed to a shoulder 16' of a shaft 2' and then radially riveted to secure the plate to the shaft. One of negative consequences of such method is poor removal of the clearances in the assembly because the stackup tolerances in the components are so large that the assembly is either pressed too tight together and, therefore, is locked up or, conversely, it is too loose to perform properly.
Pursuant to the other method, the armplate is pressed with a certain force tightly against the armplate bushing 4' and arm 1' to remove all the stackup clearances of the assembly and then is radially riveted to secure it. This approach has not been found entirely satisfactory because the press force is difficult to control and the radial rivet operation tends to push an inner area adjacent to the shaft 2' against the bushing 4' even further and can easily lock the tensioner up.